The present invention relates generally to steering systems for vehicles and more particularly to a vehicle having four wheels including a pair of front wheels and a pair of rear wheels that can be turned to produce the desired steering function for the vehicle. One type of four-wheel steering control system has first and second means that respectively control the turning movement of the front pair and rear pair of wheels.
For many years, most vehicles were controlled exclusively through the use of a steering wheel that controlled turning movements for the front pair of wheels that support the vehicle, such as an agricultural tractor. Because of the vast increase in size of these types of vehicles in recent years, manufacturers' attention has been directed towards providing other steering systems that are more versatile than the conventional front wheel steering system.
In an effort to increase the versatility of the vehicle, four-wheel steering has become an area that has received a remarkable degree of attention. For example, many steering systems have been developed wherein the respective wheels can be individually controlled to have various modes of steering control for a vehicle. U.S. Pat. No. 3,903,983 discloses a steering system for a four-wheel vehicle in which the front and rear wheels can be turned in three different modes of operation. The first mode may be referred to as the front wheel steering only, or a neutral mode in which the rear wheels are locked in a predetermined position with respect to the frame of the vehicle. The second mode of steering is where the front and rear wheels are turned an equal amount but in the opposite direction, which in commonly referred to as cramp or coordinated steering. A third mode of steering is commonly referred to as crab steering wherein the front and rear wheels are angularly disposed with respect to the longitudinal axis of the vehicle at equal angles in the same direction so that the longitudinal axis of the vehicle is moved in a generally sidewise direction.
An additional mode of operation allows for independent control of the rear wheels as well as the front wheels when the vehicle is being operated in a crowded area. The capability of having manual control over the rear wheel turning movements has been found to be very advantageous, particularly when a vehicle, such as an agricultural tractor, is being manipulated towards an implement that is normally connected to the rear end of the vehicle.
The above patent discloses a coordinated system wherein the front and rear wheels can be independently controlled or the rear wheels can be automatically positioned as a function of the angular position of the front wheels. This is accomplished through a mechanical interconnection between the front and rear wheels through a lever control system.
While this system has been found to be a commercially accepted manner of controlling the turning movements of a vehicle, such an agricultural tractor having four driven wheels, and is incorporated into large agricultural tractors that are presently being sold by the assignee of the present invention, manufacturers are constantly striving for a more versatile system which can be manufactured at a lower cost and can also be incorporated into a basic vehicle without extensive modification of the basic structure of the vehicle.
Thus, manufacturers have recently directed their development of steering systems towards other types of coordinated systems such as electronically controlled, hydraulically actuated systems. The advantage of electronically controlled hydraulically actuated systems is that such systems do not require a mechanical interconnection between the front and the rear end of the vehicle since this can normally be accomplished through the use of electrical components so that only wires need to be used as the common link between two control systems that respectively control the front and rear wheel turning movements.
While many of these systems have been proposed, very few have received any significant degree of commercial exposure. While not specifically known, it is believed that one of the reasons for the lack of commercial acceptance of electronically controlled steering systems is that many of these proposed systems do not incorporate the conventional steering wheel which has been a standard fixture in the vehicle industry almost since its inception. In addition to the conventional steering wheel that most operators still prefer over other types of steering control systems, most of these agricultural vehicles also have various other control mechanisms for performing various functions. Again, these are usually performed by manipulating some type of control member that is supported in the operator's control console.